Heat transfer enhancement by a wall-mounted flexible vortex generator with an inclination angle

Abstract

Vortex generators have been widely used in heat transfer system to enhance thermal performance by promoting fluid mixing. The convective heat flux may be improved significantly, but the installation of vortex generators into the heat transfer system is necessarily accompanied by the penalty of increasing mechanical energy loss. In the present study, two-dimensional numerical simulation is performed to investigate the effects of the self-sustained oscillating motion of flexible vortex generators on heat transfer. The wall-mounted flexible flag with an inclination angle is used to promote the fluid mixing and increase the heat transfer without much increasing the energy loss. The fluid-flexible body-thermal interaction is considered on the framework of the immersed boundary method. The influence of the inclination angle and the bending rigidity of the flexible vortex generator on the thermal performance is mainly examined. Increasing the inclination angle is observed to induce the instability of the present system, by which the self-sustained flapping motion of the flexible vortex generator favorable in terms of the heat transfer is derived. The thermal performance of the present system is significantly improved due to vortex merging process when the mode transition of the flapping motion takes place. The dynamic mode decomposition (DMD) is used to quantitatively analyze the relation between the flow structures and the temperature fluctuations. The comparison of the dynamic modes extracted from the vorticity and temperature fields indicates that the flow structures driven by the vortex generator significantly contribute to the mixing of the heated fluid near the wall with the cooled fluid passing through the center region. The temperature fields are reconstructed by using several dominant modes extracted from the present data sets.